In modern low power applications there is an ongoing need for low bias current and low supply voltage. At the core of proposed solutions are bandgap circuits. Considering a conventional bandgap implemented in CMOS technology, however, voltage supply and output voltage reference generation is subject to a basic limitation. Usually a PTAT (proportional to absolute temperature) resistor and a diode characterized by a voltage Vbe provide two temperature dependent currents which are summed with a tailored weight to provide a reference voltage independent of temperature. The voltage Vbe of a diode decreases with temperature T which is compensated for by the contribution of the PTAT element. The sum of these two contributions is to a good approximation independent of temperature T if the PTAT contribution is equal to approximately 22·Vt, in which Vt denotes the thermal voltage. The resulting reference voltage depends only on silicon properties and is slightly more than 1.2 volts in common applications.
In this conventional approach the voltage is obtained from the sum of two contributions related to circuit elements connected in series. Therefore, it is not convenient to scale down to a lower supply by using further elements connected in parallel. In particular, it is difficult and not convenient to obtain a divided reference voltage simply by arranging a parallel resistor and taking intermediate taps. Other circuits have been proposed based on mismatched pairs of diodes and a current injection approach to overcome the supply limitation. Such bandgap circuits give some more flexibility to reduce the supply voltage but demand implementation of rather large resistors. These, however, demand large area in integration decreasing the overall size of an integrated circuit.